The present invention relates to the field of optical fiber telecommunications cables and more particularly to so-called buffered fiber cables in which each optical fiber is buffered.
Cables with buffered fibers are known as “buffered optical fiber cables” and have notably been described in documents U.S. Pat. No. 3,980,390 and U.S. Pat. No. 4,629,286.
In a way known per se, an optical fiber has a central core, with the function of transmitting and possibly amplifying an optical signal, and an optical cladding, with the function of confining the optical signal in the core. The fiber also has a protective coating consisting of a photo-cross-linkable polymer material which provides a mechanical protection and seal to the optical fiber. The optical fiber, with its coating, has standardized dimensions in order to allow optical coupling with other fibers and/or connectors of optical modules. Typically, a single-mode fiber has a standardized diameter of 125 μm for the optical guide and 250 μm with the coating. A buffered optical fiber in addition to the coating has a protective buffer surrounding the coated fiber. This protective buffer is external to the coating of the fiber and is not comparable with the optical cladding of the fiber which surrounds the central core. The outer protective buffer may consist of a thermoplastic material and enable to increase the outer diameter of the fiber to a standardized value of 900 μm.
Buffered fiber cables may be used for indoor telecommunications networks. Individual access to each fiber should be possible, rapidly and easily for a distribution in a given building. For this purpose, operators make a diversion in the telecommunications cable; an opening is made in the cable and one or more fibers are picked up in order to feed a given optical system with a signal. By the increased diameter of 900 μm of the buffered fiber, handling of the thereby diverted fiber may be easier and it may be directly connected with certain optical modules.
There exist cables with tightly buffered fibers, known as “tight buffered fibers”, in which the protective buffer strongly adheres to the coating of the fiber. Such a structure is robust and remains stable over the whole temperature range of use of the cable, i.e. typically from −40° C. to +70° C. However with such a structure it is not possible to access the 250 μm fiber over more than a few centimeters. In fact, when the buffer is removed, because of the strong adherence with the coating, the coating of the fiber is also torn off leaving the optical guide exposed. This may represent a drawback when it is intended to coil up unbuffered fibers in a casing for example.
There also exist cables with semi-tightly buffered fibers, known as “semi-tight buffered fibers”, in which the protective buffer is decoupled from the coating of the fiber by an interstice of air or filling gel for example. With such a structure, it is possible to remove the protective buffer without damaging the coating of the fiber but this structure is not stable over the temperature range of use of the cable. Indeed, the thermoplastic material of the protective buffer is subject to deformations due to changes in temperature, which introduces an axial stress on the buffered fiber and causes an increase in attenuation. The contraction of the protective buffer under the effect of a drop in temperature may thus introduce a “piston effect” at the end of the diverted fiber which makes any operation delicate for connecting it to an optical module.
The decoupling of the protective buffer and of the coating of the fiber also poses a problem during diversion operations. To divert a buffered fiber, the operator pulls the fiber through a diversion window; this tension force is applied on the protective buffer which may lengthen elastically and cause a shift between the length of the diverted optical fiber relatively to the length of the drawn protective buffer. Such a shift also causes attenuation in the optical signal transmitted by the fiber.
Therefore, it was sought to make buffered optical fibers with a protective buffer which is sufficiently coupled to the coating of the fiber in order to avoid any shift, but which may be easily removed without damaging the coating so as to allow access to the 250 μm fiber over at least one meter.
Document U.S. Pat. No. 5,181,268 describes a buffered optical fiber comprising an intermediate layer between the coating of the 250 μm fiber and the 900 μm protective buffer. With the intermediate layer, friction may be reduced between the protective buffer and the coating in order to facilitate removal of the protective buffer without damaging the coating. This document proposes an intermediate layer consisting of a solid lubricant and a binder. The lubricant may be TEFLON and the binder may be an acrylic polymer.
The intermediate layer proposed in document U.S. Pat. No. 5,181,268 cannot be extruded in tandem with the protective buffer but requires a specific application. It is notably necessary to have the fiber coated with the intermediate layer passed into a drying oven before adding the outer protective buffer. The intermediate layer of document U.S. Pat. No. 5,181,268 complicates the cable manufacturing method and increases its price. In addition, TEFLON contains fluorine (polytetrafluorethylene) and is therefore not a halogen-free material. Now, certain legislations, and notably European legislation, impose the sole use of halogen-free materials for indoor installations. The cable of document U.S. Pat. No. 5,181,268 would therefore not be suitable for cabling a building, in compliance with these regulations.
Document EP-A-0 690 033 also describes a buffered optical fiber comprising an intermediate layer between the coating of the 250 μm fiber and the 900 μm protective buffer. This document points out that the intermediate layer of U.S. Pat. No. 5,181,268 does not sufficiently adhere to the coating of the fiber, does not allow line manufacturing at high speed and is not sufficiently uniform. In order to solve these identified problems, document EP-A-0 690 033 proposes a cross-linked intermediate layer comprising ultra high density polyethylene (UHMWPE) or TEFLON mixed with a photo-cross-linkable binder such as urethane polymer.
Document U.S. Pat. No. 6,775,443 also describes a buffered optical fiber comprising an intermediate layer between the coating of the 250 μm fiber and the 900 μm protective buffer. This document proposes a cross-linked intermediate layer comprising an urethane acrylate matrix comprising oligomers, monomers, a photoinitiator and an anti-oxidant in combination with a liquid reactive release substance, such as liquid silicone.
The intermediate layers proposed in documents EP-A-0 690 033 and U.S. Pat. No. 6,775,443 should be cross-linked; their passing under UV lamps should therefore be foreseen and application of the intermediate layer cannot be easily tandemized with the extrusion of the outer protective buffer. Further, photo-cross-linkable materials are relatively expensive.
There is therefore a need for a buffered optical fiber comprising an intermediate layer which may be applied without slowing down the fiber manufacturing method and having limited cost. The intermediate layer should guarantee proper coupling between the coating of the fiber and the outer protective buffer in order to avoid any shift of the buffer relatively to the fiber, without providing mechanical stresses on the fiber and should allow the protective buffer to be removed rapidly and easily without damaging the coating of the fiber.